along the shore of the lake. Evidence 

 that phosphorus loading has increased 

 over time is shown in Figure 44. This 

 nutrient loading is similar to the pat- 

 tern in Lake Salvador (Craig and Day 

 1977). 



The average rate of primary pro- 

 duction in Lake Pontchartrain does not 

 reflect nutrient loading, however. For 

 example, Dow and Turner (1980) estimated 

 that only about 160 g C/m 2 is fixed in 

 the water column of Lake Pontchartrain 

 annually, compared with 220 g C/m 2 for 

 Lake Salvador. If this difference is 

 real, it may indicate that primary pro- 

 duction in Lake Pontchartrain is being 

 inhibited. On the other hand, excess 

 nutrients may precipitate upon entering 

 the lake, as seems to occur in Lake 

 Salvador. Two possible explanations for 

 inhibition of primary production in Lake 

 Pontchartrain have been suggested. The 

 first is that the turbidity of the lake 

 may have increased recently, partly as a 

 result of shell dredging (see below). 

 Another possibility is that aquatic 

 producers may be inhibited to some 

 extent by herbicides, such as 2,4,D and 

 2,4,5,T, that have been detected in the 

 lake water (Sikora et al . 1982). 



There is some evidence for a recent 

 increase in turbidity in the water of 

 Lake Pontchartrain. One study suggests 

 that the lake is on the average about 

 twice as turbid as it was in 1953 (Stone 

 1980a). This thesis is based on overall 

 minimum Secchi disc readings taken 

 during four different studies beginning 

 in 1953, which seem to indicate a clear 

 trend (Figure 45). The possible effects 

 of variable wind conditions during these 

 studies have not been factored into this 

 plot; however, it is assumed that none 

 of the readings were taken during high 

 wind conditions. 



A combination of land clearing and 

 increased sedimentation in the Pont- 

 chartrain basin, intensive clam shell 

 dredging, in the lake proper during the 

 last 30 years, along with the six Bonnet 

 Carre floodway openings, could perhaps 

 account for such a turbidity increase. 



Another piece of information that 

 relates to the rate of primary produc- 

 tion in Lake Pontchartrain was presented 

 by Roberts (1931) and Roberts and Bahr 

 (1981). A set of aerobic respiration 

 measurements of lake bottom samples 

 indicated that the minimal carbon 

 requirements of the benthic community 

 may be greater than the estimated rate 

 of primary production in the water 

 column. This observation, if borne out, 

 would indicate the dependence of the 

 lake on carbon washed into the lake from 

 its watershed. 



The present condition of Lake 

 Pontchartrain was recently characterized 

 in a study funded by the U.S. Army Corps 

 of Engineers (USACE) (Stone 1980a and 

 b) . During this study, and in a subse- 

 quent related study on the effects of 

 shell dredging on the lake, the benthic 

 community was examined in detail (Bahr 

 et al. 1980; Roberts 1981; Roberts and 

 Bahr 1981; Sikora et al. 1981; Sikora 

 and Sikora 1982). The conclusion from 

 these studies is that the benthic com- 

 munity of the lake is symptomatic of a 

 highly stressed ecosystem. There is an 

 unusually small number of organisms, low 

 biomass, and low species richness, 

 especially when compared to previous 

 research in the lake (Darnell 1979). For 

 example, only about seven species of 

 macrofauna occurred in most samples, and 

 practically no large brackish water 

 clams ( Rangia cuneata ) were found in the 

 open lake where they had been thriving 

 as recently as 1953 (Darnell 1979). 



One of the major alterations to 

 Lake Pontchartrain is the continuing 

 mining of fossil Rangia shells from lake 

 sediments. Hydraulic shell dredging is 

 allowed in almost 50% of the lake bottom 

 area, which has been estimated to be 

 completely scoured at least once every 

 1.4 to 2.3 years (Sikora et al. 1981). 



In a recent study of the effects of 

 this dredging on the lake system (Sikora 

 et al. 1981) it was found that the bot- 

 tom sediments are destabilized so much 

 that they are too soft to support adult 

 clams. Because these clams are dredged 



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